Geological depositions of oil sand, also known as tar or bituminous sand, occur for example in the Athabasca region of Alberta, Canada.
Commercial processes for extracting and refining the bitumen to yield useful hydrocarbon products from the oil sand have long been in operation.
In these operations, the basic procedure involves removing the overburden and mining the oil sand. The hydrocarbon is then extracted from the oil sand utilizing a process known as the hot water process. The recovered hydrocarbon is upgraded in a hydrotreating facility to convert it to a refineable product.
It is the physical nature of the oil sand per se which renders it amenable to successful processing using the hot water extraction process.
The composition of oil sand comprises bitumen, water, quartz sand and clays. The quartz sand forms the major component. The clay particles are contained in a water matrix which forms a film around each sand grain. The bitumen is disposed in the interstices between the watersheathed grains. The presence of the water envelope, interposed between the hydrocarbon globules and sand grains, provides the basis whereby the bitumen may be separated from the sand by means of a water addition mechanism.
In order to successfully carry out the hot water process, it is necessary to first separate the bitumen from the solids particles and then selectively aerate the bitumen globules so that the latter float upwardly as a recoverable upper froth layer.
Thus the process relies on the density differentials within an aqueous slurry of the solids, water and bitumen, and the use of a selective separatory froth flotation process wherein the solids sink and the bitumen rises to form the froth.
More specifically, the first step of the hot water process involves an operation referred to as `conditioning`. In this step, the mined oil sand is mixed in a horizontal rotating drum, or `tumbler`, with hot water and process aid (typically sodium hydroxide). The amounts of reagents added are in the following proportions: oil sand--3250 tons; hot water--610 tons; and NaOH--4 tons. The hot water is typically at a temperature of about 90.degree. C. Steam is sparged into the drum contents at intervals along the length thereof to trim the temperature so that the slurry exit temperature is about 80.degree. C. The residence time in the drum is about four minutes.
The conditioning operation is undertaken for several reasons. The water is added to displace the bitumen and solids particle away from each other. The hot water and steam cooperate to raise the temperature of the slurry. This will lower the viscosity of the bitumen and thus enhance its displacement from the solids by water. The higher temperature also increases the density differential between the bitumen and water. This facilitates the separation therebetween in the subsequent flotation/separation stage which follows conditioning. Additionally, as the slurry undergoes agitation in the tumbler, beneficial entrainment of air bubbles therein results.
Following conditioning, the thick aqueous slurry is screened to remove rocks, oversize oil sand and clay lumps. The screened slurry is then diluted or `flooded` with additional hot water before being pumped into the flotation/settling vessel (commonly referred to as the `primary separation vessel` or `PSV`). The thus diluted slurry will be referred to hereinafter as `the diluted aqueous slurry`. The slurry at a point prior to its dilution will be referred to hereinafter as `the slurry`.
The composition of the diluted aqueous slurry typically comprises 7% wt. bitumen; 43% wt. water; and 50% solids.
The diluted aqueous slurry is then pumped into the PSV. This open-topped vessel comprises a cylindrical upper section and a conical lower section. The aqueous slurry is retained in the PSV under quiescent conditions for a period of time, typically in the order of twenty-five minutes. The solids, largely sand, sink to the vessel bottom, are concentrated by the conical wall, and are withdrawn from the vessel as an underflow stream termed `primary tailings`. A major portion of the bitumen, present in the form of suspended globules filmed over entrained air bubbles, rises rapidly to the top of the PSV to form bitumen-rich froth. This froth is termed `primary froth`. Primary froth typically has a hydrocarbon content in excess of 60% wt.
Less buoyant or inherently less floatable bitumen, together with a substantial portion of the clay particles, remains in aqueous suspension between the settled sand and the floating froth layers. This suspension is referred to as `middlings`. The aqueous phase of the suspension is termed `process water`.
The hot water process further includes a secondary recovery circuit. More particularly, a stream of middlings is withdrawn from the PSV and passed through one or more serially connected sub-aerated flotation cells. The middlings are subjected therein to vigorous agitation and aeration. Bitumen-rich froth, termed `secondary froth`, is produced and recovered from the upper surfaces of the cells. The recovered secondary froth, usually having a hydrocarbon content of about 25%, is subsequently retained in a settling tank for a period of time to allow some contained water and solids to settle out. The remaining `cleaned` secondary froth is then admixed with the primary froth to produce a combined froth product.
The secondary froth is considerably more contaminated with water and solids than is the case with the primary froth. More particularly, the primary froth might typically contain about: 66.4% wt. bitumen; 8.9% wt. solids; and 24.7% water. The secondary froth typically might contain about: 23.8% wt. bitumen; 17.5% solids; and 58.7% water.
It is, therefore, an objective in the operation of the hot water process to seek to maximize the recovery of the bitumen contained in the oil sand in the form of primary froth. That is to say, it is desirable that the bitumen report as primary froth rather than secondary froth. One also seeks always to maximize total bitumen recovery.
Before the combined froth can be advanced to the upgrading operation, it is first necessary to remove most of the water and solids therefrom. This is conventionally accomplished by means of a two-stage centrifuging circuit. In this circuit, the combined froth stream is first diluted with naphtha and then fed to a scroll centrifuge to separate off the bulk of the coarse solids. The product stream, comprising water, bitumen and fine solids, is then passed through a high-speed disc centrifuge to recover the bitumen.
Whilst many of the hot water process parameters have heretofore been extensively researched, relatively little research has been directed to the addition of air and its effects on primary froth recovery. This omission was perhaps a consequence of early development work, undertaken by the present assignee. Air injected into the primary separation vessel had resulted in an increase in the contamination by solids and water of the primary froth. Additionally, researchers had injected air into the tumbler, without finding any significant increase in primary froth recoveries. At the time of the present invention it was, therefore, the widely held belief in applicants' laboratory that air addition (as opposed to air entrainment in the tumbler) was either mildly deleterious in the process or was not a critical parameter either way.